1. Field of Invention
The present invention is directed to detectors for measuring concentrations of a gaseous sample, and more particularly directed toward a pulsed discharge detector that correlates collected data with the discharge and calibrates collected data to reduce effects of discharge noise.
2. Related Art
Various forms of detectors are used to quantify constituents of a sample gas. Known detectors vary in how they make the sample detectable, arrange the necessary components of the detector, actually detect characteristics of the sample and process the detected data.
Pulsed discharge detectors ionize a discharge gas in a chamber to produce photons, introduce a sample gas to be tested and measure ionization currents of electrons produced from interaction of the photons with the sample gas. The ionization source of the detector is an electrical discharge in a discharge gas. The discharge gas may be a noble gas or a combination of noble gases.
As a test instrument, a pulsed discharge detector may be provided with a sample input gas eluted from a gas chromatograph column or other suitable source. The column effluent normally includes a carrier gas which is routinely input through the column at a specified flow rate. The column elutes the various constituents in peaks of concentration in a specific timed sequence, dependent on volatility of the sample constituents.
The gas chromatograph column separates compounds but does not quantify the concentrations of the compounds. Gas chromatograph detectors are connected downstream the column for quantitative analysis. By using a series of calibration gases, a fixed flow rate, and a specific stationary phase material, the gas chromatograph column can be used to separate compound types based upon the retention time. There may be any number of eluted peaks formed by the gas chromatograph column output to be quantified.
The ionization mechanism of a pulsed discharge detector is primarily photo-ionization wherein an electric discharge generates diatomic molecular emissions of photons. The high-energy photons in turn ionize the sample compounds in the detector chamber. When the pulsed discharge detector uses helium as the discharge gas, the photon generation process includes the following steps:                1. The electrical discharge ionizes some helium atoms, He, to helium ions, He+.        2. The He+ ions combine with helium atoms, He, to form diatomic molecular ions, He2+.        3. Each diatomic ion, He2+, captures one electron, dissociating back to two helium atoms, 2 He, emitting photons in the process.During step 3, a continuous photon emission arises from the molecular interaction—the transition of diatomic helium molecular ions to ground state helium. These photons have an energy level in the range of 13.5˜17.5 eV, which can ionize almost all compounds, except helium itself. Other processes of molecular interaction may also affect the electrical discharge, such as helium atomic emission and helium meta-stable generation.        
Pulsed discharge detectors possess favorable characteristics over other gas chromatograph detectors. First, sensitivity is higher. The minimum detectable limit of gases present in a sample using a regular pulsed discharge detector in helium ionization detector mode is about 10 times lower than the minimum detectable limit identifiable using a flame ionization detector. Pulsed discharge detectors are operable to determine concentrations at the parts-per-billion level. Second, pulsed discharge detectors offer selectivity in response. Pulsed discharge detectors have a universal response when helium is used as the discharge gas. When helium is doped with another noble gas as the discharge gas, pulsed discharge detectors may have a selective response. Third, pulsed discharge detectors have a uniform response factor. Within an organic group the response factor increases linearly with the carbon number of the sample. Fourth, the pulsed discharge detector system does not require use of radioactive material.
Wentworth, et al. U.S. Pat. No. 5,394,091, teaches an ionization detector adapted for use in either helium ionization or electron capture mode. The detector utilizes a helium flow through a detector cell or chamber. The chamber has regions of spark discharge, sample introduction and sample detection. The helium flow is the only flowing material in the immediate region of the spark. A sample gas and/or carrier gas are injected and commingled with the helium gas downstream from the spark in the sample introduction region. Two electrodes, of which one is bias, detect the charged characteristics of the sample, the bias electrode at or upstream of the sample inlet and the other electrode downstream from the sample inlet. An electrometer measures the difference between the resulting currents. These current measurements are recorded on a timed basis. A base line current is formed as a result of impurities in the discharge or carrier gas.
Stearns, et al. U.S. Pat. No. 5,767,683, teaches a pulsed discharge detector having a bias voltage feedback system. The feedback system compares output from an electrometer connected to the collector electrode with a reference current using a comparison circuit. Output from the comparison circuit is input to a control circuit, which, in turn, outputs a bias voltage. The bias voltage is applied to the first bias electrode such that the electron current flow within the detector chamber remains constant for all concentrations of input sample gas. The instantaneous setting of the control circuit is used to form a second output. The magnitude of this second output is proportional to the concentration of a selected sample gas within the electron capture detector chamber and is the response signal of the disclosed electron capture detector system. Pulsed discharge detectors provide intermittent electrode discharges. A typical discharge voltage is in the range of 300 to 400 volts. A typical time interval between discharges is in a range of 100 microseconds to 800 microseconds.
Current pulsed discharge detectors measure, on a continuous basis, the current output of the sample gas, including readings during discharge events and including readings during the interval between pulses.
Sensitivity of measurement results is adversely affected by discharge variations, referred to as discharge noise. Discharge noise can be reduced by a high quality pulser and a clean discharge electrode surface, but cannot be eliminated.
In prior art pulsed discharge detectors, a relatively short discharge period (interval between discharges) increases the sensitivity of the measurement. However, a relatively short interval between discharges results in increased average electrode temperatures and relatively short electrode life cycle.
The present invention provides an improvement to the prior art by increasing sensitivity of the pulsed discharge detector by providing a device and method to measure ionization signals in the sample gas modularly at the time of the discharge event as opposed to a collected current, by a geometric configuration that enhances collection of signals and by providing a means of calibrating collected signals to account for discharge noise.
The present invention accordingly provides an improvement to the prior art by allowing longer pulse periods, thereby reducing power requirement of a pulsed discharge detector and providing relatively longer product life.
The reduced power requirements of the detector of the present invention makes the detector particularly useful as a portable detector.